Heat treating of steel



0d. 20, 1942. 1 R GIER1 JR 2,299,138

HEAT TREATING OF STEEL Filed l001. 4, 1941 2 sheets-sheet 1 m ,C1-g 1f,` figa 5913.

u Q 700 Y 1 I PAQ /e Y?? f o 0 .01o .02o .03o .04o o .010.920 .0,70 .Mo o .alo .ozo .oso .04o

g k Ik j Dep/h e/owurface fnCh) WCA/ef; Hard/)eff (/OA/y/aaaf) Oct. 20, 1942. 1 R GlER, JR 2,299,138

HEAT TREATING OF STEEL .0/0 .020 .DJ 0 .O10 ,020 .030 .046 0 .0/0 ,020 .030 .040

.Dep fh Be/aw fur/ace (fn cb) Patented Oct. 20, 1.942

HEAT 'IREATING oF STEEL John R. Gier, Jr., Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a. corporation of Pennsylvania Application October 4, 1941, Serial No. 413,625

' (ci. '14s-17) 6 Claims.

This invention relates to the h'eat treating of steel, and particularly to a method of hardening stainless steel.

This application is a continuation-in-part of my copending applications Serial Nos. 296,586 and 296,587, which were led September 26, 1939, entitled Heat treating furnaces, now issued as Patents Nos. 2,290,551 and 2,290,552, respectively, and which are assigned to the same assignee as this invention.

Many types of mechanical devices operate in such a way that failure, corrosion or wear on one or more surfaces may cause serious functional breakdown of th'e mechanism. Particularly in outdoor or marine applications, it is necessary to provide certain mechanical devices which are maintained free from rust, have suicient hardness to lresist wear, have suicient .impact strength vto resist chipping or breaking, and have a uniformity of frictional coeicient.

Heretofore, different methods of heat treating metals have been employed to give them improved characteristics. Nitriding of steel has greatly extended the range of service life on many wearing parts because of the combination of unusually high hardness and wear resistance characteristics which are obtainable while at the same time obtaining moderate corrosion resistance and low frictional characteristics. However, where the nitrided steels are employed wh'ere concentrated loads or impact stresses are encountered,par ticularly at edges and sharp corners, chipping and spalling of the nitrided case are encountered. Other known processes, such as case carburization, have not been employed to give toughness to the steel except where corrosion resistance is not a factor.

In marine or seaboard service, it is often'necessary to employ stainless steel in' many of the applications because of its corrosion resistance. This corrosion resistance of the stainless steel is provided throughout the cross-section of the part formed therefrom but is accompanied by a decrease in wear resistance. The stainless steel parts invariably have low uniform frictional characteristics rendering them especiallyl suitable for vapplications such as engaged latches which must taneously eiect the case carburization and the bright hardening of stainless steel.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which:

Figures l, 2, 3, 4, 5 and 6 are graphs, the curves of which illustrate the eiect of variations in the composition of the gaseous atmosphere on the hardness gradient of the steel treated in accordance with this invention;

Figs. 7, 8 and 9 are graphs, the curves ofA which illustrate thereiect of time and temperature of treatment on the hardness gradient of another steel treated in accordance with this invention;

Figs, 10, 11 and 12 are graphs, the curves of which illustrate the effect of time and temperature of treatment on the hardness gradient of a different steel treated in accordance with th'is invention; and v Fig. 13 is a. graph,ithe curves of which illustrate the effect of variations in the gaseous atmosphere on the hardness gradient of the stainless steel treated in accordance with this invention.

In practicing this invention, excellent results are obtained where a protective atmosphere, formed of extremely pure dissociated ammonia to which from 0.4% to 2% of methane is added, is employed. Good results have also been obtained where up to 3% of methane is added to the dissociated ammonia in forming the protective atmosphere.

vThe method of this invention is particularly applicable to stainless steel articlesv having a.l

chromium content of 10 to. 15% and a carbon content of 0.5 to .90%. It is especially applicable to stainless steel articles h'aving a preferred composition of 11.5% to 14% chromium, .10% to .36% carbon, less than .5% of each of manganese,

silicon and phosphorus, with the balance iron, except for incidental impurities.

In practicing this invention, the steel articles having the composition given hereinbefore are subjected to a heat treatment which' normally eiects the bright hardening thereof. This treatment consists preferably of Aheating the stainless steel articles in a protective atmosphere at a temperature between 825 C. and l040 C.- for a predetermined period of time of between 15 minutes and three hours, and effecting an atmospheric quenching or cooling of the articles in the protective atmosphere from the heating temperature.

The methane added may be pure methane or may be added in the form of natural gas which, as is Well known, contains about of methane,

'I'he heat treatment of the stainless steel articles is preferably done in a furnace of the type disclosed and claimed in my Patent No. 2,290,552 which' issued July 21, 194,2, entitled Heat treating furnaces, and in which provision is made for withdrawing the articles from the heating chamber to a quenching or cooling chamber in which the articles are caused to be quickly cooled while enveloped in an atmosph'ere of thesame compositio: as the protective atmosphere within the heating chamber. Other furnaces, and particularly furnaces of the type disclosed in the cepending application of W. E. Mahin, Serial No. 416,181, filed October 23, 1941, may, however, be utilized as long as provision is made for obtaining and admitting the mixture of pure dry dissociated anunonia and methane, th'e mixture having a low dew point of 60 C. to' 80 C., into the heating chamber and the quenching chamber and preventing contamination of the protective atmosphere withinthe furnace. In practicing this invention, it is important tomaintain th'e low dew point of 60 to 80 C. for the purpose of bright finishing the steel and for maintaining the -carburizing power of the protective atmosphere.

When the stainless steel articles are treated in the protective atmosphere as described, it is found that a simultaneous bright finishing and case carburization of the articles is effected. The treated stainless steel has an extremely bright,r surface, a definite case hardness and a very tough core.

The hardness of the case and the depth of the case are affected by the amount of methane mixed with the dissociated ammonia, the temperature at which the steel is treated and the length of time of such treatment, as is clearly illustrated by the curves of Figs. 1 through 13 of the accompanying drawings.

Referring to Fig. 13,- there is illustrated the hardness gradient curves obtained on a ,30% carbon type of stainless steel having the specific analyses of .36% carbon, 13.96% chromium, 0.34% manganese, 0.12% silicon', 0.02% phosphorus and the Abalance iron and incidental impurities when subjected to the heattreatment consisting of heating the steel at a temperature of 1010 C. for one hour in the protective atmosphere, then quickly cooling the steel while enveloped and protected by the protective atmosphere. Curve I illustrates the hardness gradient obtained where .05% of natural gas, equivalent to 0.42% methane, is added to the dissociated ammonia to form the protective atmosphere while curve I2` illustrates the hardness gradient obtained with a steel of the same analyses and treated in the same manner, except that 3.0% of natural gas, equivalent to ,2.55% methane, is added to the dissociated ammonia to form the protective atmospliere.4 As illustrated, the increase in the methane content of the protective atmosphere increases the case hardness imparted to the stainless steel without detractng from the toughness of the core.

Referring to Figs.v 1, 2, 3, 4, 5 and 6, there is illustrated the effect ofadditions of methane to the protective atmosphere on the case hardness of a 0.10% carbon type stainless steel having the specific analyses of 0.12% carbon, 13.32% chromium, 0.30% manganese, 0.56% silicon, 0.01% phosphorus, and the balance iron and incidental .The methane is preferably mixed with th'e pure dissociated ammonia prior to being admitted to impurities. m'these ngures. curves u, is, u,

20, 22 and 24 of Figs. 1, 2, 3, 4, 5 and 6, respec` tively, illustrates the eiect of variations of the methane content ofthe protective atmosphere on the hardness gradient. The hardness gradient illustrated by curves I4, I 6, I8, 20,' 22 and 24 is obtained where 0.5%. 1%, 1.5%, 2%, 2.5% and 3%, respectively, of natural gas is added to the dissociated ammoniawith the heat treating cycle comprising heating the steel at a temperature of 1010 C. for a period of one hour, after which the steel is cooled while'enveloped in the respective protective atmospheres. Curves, 28, 30, 32, 34 and 36 of Figs. 1 through 6, respectively, illustrate the hardness gradient obtained where the steel is heated at a temperature of 1010 C. for a period of one hour in the dissociated ammonia protective atmosphere containing natural gas additions in the Aamount of 0.5%, 1%, 1.5%, 2%, 2.5% and 3%, respectively, and cooled in the furnace atmosphere, after which the steel is after cooling in dry ice to effect at least a partial transformation of the austenite. An examination of the results illustrated in Figs. l through 6 reveals that where this particular steel is treated in a protective atmosphere containing more than 1% methane, excessive amounts of retained austenite are obtained resulting in low hardness at or near the surface of the stainless steel article.

Referring to Figs. 7, 8 and 9, there is illustrated the effect of the heat treatment of this invention on the hardness'gradient curves for a 0.30% carbon type stainless steel having the specific analyses of 0.36% carbon, 13.96% chromium, 0.34% manganese, 0.12% silicon, 0.02% phosphorus, and the balance iron and incidental impurities. These curves primarily show the effect of time'and temperature of treatment on the hardness gradient, the curves of Fig. 'lf illustrating the hardness gradient when the steel is subjected to the heat treatment at different temperatures for 1/2 hour, the curves of Fig. 8 illustrating the effect of the treatment at different temperatures for a period of time of 1 hour, and the curves of Fig. 9 illustrating the effect of the treatment at different temperatures for a period 0f time o'f 2/2 hours. In all cases, the heat treatment was effected in a protective atmosphere consisting of pure dissociated ammonia containing 1% of natural gas, which is equivalent to .85% of meth-ane.

In the drawings, curves 31, 38 and 39 of Figs. '7, 8 and 9, respectively, illustratev the hardness gradient for this particular steel when the heat treatment includes heating the steel at a temperature of 900 C. Curves 40, 4I and 42 of Figs. 7, 8 and 9, respectively, illustrate the hardness gradientobtained where the heating temperature is .950 C. Curves 4I, 44 and 45 of Figs. 1, 8 and 9, respectively, illustrate the'hardness gradient on the same steel when the heat treatment includes heating at a temperature of 1000 C., while curves 46, 41 and 48 of Figs. '7, 8 and 9, respectively, represent the hardness gradient of the steel when the steel is subjected' to a treatyasesinas ment including heating at a temperature of 1040 C. From the curves of Figs. '7, 8 and 9, it is evident that for a given temperature, the maximum hardness increases with time at the lower temperatures but at the higher temperatures austenite retention results in the dvelopment of low surface hardness.

Referring to Figs. 10, 11 and 12, there is illustrated the effect of time and temperature ofthe heat treatment of this invention on the hardness gradient curves for a 0.10% carbon type of stainless steel having the specific analyses of 0.12% carbon, 13.32% chromium, 0.30% manganese, 0.56% silicon, 0.01% phosphorus and the balance iron and incidental impurities. 'I'he curves of Fig. are based on steel which has been subjected to a treatment in dissociated ammonia containing .85% methane for a period of time of l/2 hour at a heating temperature, whereas the curves of Figs. 11 and 12 were obtained for treatments in the same protective atmos-I phere at heating temperatures for a period of time of 1 hour and 21/2 hours. respectively. v

In the drawings, curves 49, 50 and 5l of Figs. 10, l1 and 12, respectively, are'representative of the hardness gradient obtained where the heat treatment included heating at a temperature of 900 C.. while curves 52, 53 and 54 of Figs. 10, 11 and l2, respectively, represent the hardness gradient obtained where theheat treatment included heating at 950 C. Curves 55, 56 and 51 of Figs. l0, ll and 12, respectively, illustrate the hardness gradient for the different periods of time of heat treatment at a temperature of 1000 C.. while curves 58, 59 and 60 of Figs. 10, 11 and 12, respectively, are representative of the results obtained where the heating temperature was l 040 C. The curves of Figs. 10, 11 and l2 illustrate the same tendencies as the tests in which the amount of methane in the atmosphere is varied, that is, when the carbon at the surface reached a concentration resulting ina hardness of approximately 700 Vickers austenite retention begins to occur, and further carburization causes lower hardness. This is particularly true at the higher temperatures which promote austenite retention. v

The heat treatment of this invention improves the transverse breakingA strength of the stainless steel articles. For example, a stainless steel having a carbon content of 0.37% and a chromium content of 12.6% by analyses has a breaking strength of 189,000 lbs. per square inch after having been treated for one hour at a temperature of 1010 C. in pure dissociated ammonia. When the same steel is heated for the same period of time, and at the same temperature'in the protective atmosphere utilized in the method of this invvention consisting of dissociated ammonia and 1% of methane, the breaking strength is increased to 260,000 lbs. per square inch. At the same time, the Rockwell C hardness of the steel after being subjected to the respective treatments, is 58 and 62, respectively, the higher hardness being obtained through the use of the protective atmosphere containing methane.

As another example of improvement in the hardness of steel when treated inaccordance with this invention, the surface hardness of the lowy carbon (0.12%) martensitic stainless steels is increased to as high as 66 Rockwell C with no apparent decrease in corrosion resistance when heat treated in the protective atmosphere of dissociated ammonia and from 1 to 2% of free methane thereby providing a high surface hardness and a tough, impact resisting, core.

The method of this invention is eifective for simultaneously producing a bright finish and case` carburization of the stainless steels. This method finds particular application in the bright hardening of stainless steel articles such as springs, triggers, latches, levers, pins, small gears, or the like, which are subjected to impact stresses, and at the same time provides articles having corrosion resistance, excellent wear resistance, freedom from spalling and marked uniformity of frictional coeicient.

I claim as my invention:

1. In the method of case hardening stainless steel containing from .05% to .90% carbon and 10% to 15% chromium, in combination, the step of heating the steel in an atmosphere comprising dissociated ammonia and` from 0.4% to 3% of methane.

2. In-the method of case hardening stainless steel containing from .05% to .90% carbon and 10% to 15% chromium, in combination, the step of heating the steel in an atmosphere comprising dissociated ammonia and about 1% of methane.

3. 'I'he method of claim 1 in whichthe steel is heated fora period of time of from 15 minutes to 3 hours at a temperature between 825 C. and 104o c.k

4. In the method of case hardening stainless steel containing from .05% to .90% carbon and 10% to 15% chromium, in combination, the steps of, heating the steel in an atmosphere comprising dissociated ammonia and from 0.4% to 3% of methane at a temperature between 825 C. and 1040 C., and cooling thesteel in the atmosphere containing methane.v

5. The method of case hardening stainless steel containing from .05% to .90% carbon and 10% to 15% chromium comprising, heating the steel in an atmosphere comprising dissociated ammonia and from 0.4% to 3% of methane for a period of timev of from 15 minutes to 3 hours at a temperature between 825 C. and 1040 C., and cooling the steel in the atmosphere of dissociated ammonia and methane to provide a hard outer case on the steel free from tarnish or scale.

6. In the method of heat treating stainless steel containing from .05% to .90% carbon and 10% to 15% chromium, in combination, heating the steel in a gaseous atmosphere comprising dissociated ammonia and from 0.4% to 3.0% meth- JOHN R. GIER, JR. 

